Lithium niobate (LN) has emerged as a cornerstone material in the fields of nonlinear optics and integrated photonics. While traditional bulk lithium niobate crystals have been extensively utilized for laser frequency conversion, they face significant limitations due to the walk-off effect and weak light field confinement during the process. These challenges necessitate extremely high pump light power levels to achieve substantial nonlinear conversion efficiency, making them less suitable for on-chip integration applications. The development of lithium niobate-on-insulator (LNOI) thin films has addressed these limitations through advanced bonding and thinning processes. LNOI not only retains the advantageous properties of bulk lithium niobate crystals but also offers a high refractive index contrast and the micron-scale light field confinement capability. Although Ti-diffused or proton-exchanged waveguides can achieve high frequency conversion efficiency, their relatively low damage thresholds (below 100 MW/cm2) significantly limit their applicability in high-power scenarios. In contrast, ridge waveguides directly fabricated in LNOI thin films through etching or other manufacturing techniques not only avoid introducing lattice defects but also maintain the excellent physical properties of the bulk crystals. While nanoscale thin film waveguides demonstrate ultra-high normalized conversion efficiency, their submicron mode field dimensions result in significant mismatch with standard fiber modes, leading to excessive coupling losses. To realize commercially viable high-reliability and high-power frequency conversion devices, the use of micron-scale thin film waveguides is imperative. The micron-scale LNOI ridge waveguides offer a promising balance between efficient nonlinear interaction and practical coupling performance, making them particularly suitable for demanding high-power applications. Further research should focus on optimizing their structural design and fabrication processes to fully unlock their potential in integrated photonics systems.

As illustrated in Fig. 1(a), the ridge waveguide utilized in this work features a ridge height of 7 μm, a ridge width of 7 μm, an etching depth of 3.5 μm, and sidewall inclination of 70°. The waveguide length is 15 mm, with a poling period of 18.18 μm for the 1560 nm second-harmonic generation (SHG) process. Our platform is based on z-cut LNOI thin film, comprising a 7 μm-thick LN layer, a 2 μm-thick SiO? layer, and a silicon substrate. The fabrication process involves two key steps: the periodically poling and the waveguide etching. As detailed in Fig. 2(a), the procedure begins with the deposition of aluminum layers on both sides of the LNOI wafer using electron beam evaporation. A periodic poling electrode pattern is then defined through spin-coating lithography, followed by the wet etching to reveal the electrode areas. Following the completion of the periodic poling process, a chromium layer is plated onto the sample surface to serve as a mask. The waveguide path is subsequently defined using spin-coating lithography, and the ridge waveguides are etched using the inductively coupled plasma. After completing the inductively couple plasma etching process, the chromium mask is removed using a chromium etchant solution. Finally, the waveguide sidewalls are polished to reduce the transmission loss, and both end facets of the waveguide are polished and coated with an anti-reflection layer to minimize the Fresnel reflection loss.

Under low-power conditions, the normalized conversion efficiency of the SHG waveguide is measured to be 61%/(W·cm2). The relationship between the output power of the frequency-doubled light and the input power of the fundamental light follows a quadratic dependence. As the input power of the fundamental light increases, the output power of the frequency-doubled light scales linearly with it. Ultimately, an output of frequency-doubled light at 780 nm exceeding 5 W is achieved (Fig. 5). The internal conversion efficiency of the waveguide is determined to be 84.8%, while the overall device conversion efficiency reaches 52.6%. Analysis reveals that the residual fundamental light in higher-order modes and the temperature gradient generated by thermal effects at high power are the primary factors limiting the internal conversion efficiency from reaching 100%. Finally, a 24 h high-power stability test is conducted, which demonstrates excellent stability with the output power fluctuation within ±3%.

This paper presents a high-power periodically poled lithium niobate (PPLN) waveguide frequency conversion device with an all-fiber structure. Based on the 7 μm-thick magnesium-doped LNOI thin films, the PPLN ridge waveguides are fabricated using dry etching technology. Under an input power of 9.5 W at the fundamental wavelength of 1560 nm, the device achieves a SHG output of 5 W at 780 nm, corresponding to an overall conversion efficiency of 52.6%. Notably, after continuous operation for 24 h under high-power conditions, the SHG output power fluctuation remains within ±3%. Additionally, this study experimentally identifies the higher-order modes of the fundamental light as the principal factor constraining the device conversion efficiency from reaching its theoretical limit. The proposed single-mode condition provides a clear guideline for optimizing waveguide dimensions to enhance the frequency conversion efficiency. This high-power PPLN waveguide frequency conversion device successfully combines the high conversion efficiency with the low fiber coupling loss, delivering a significant output power. These advancements support the development of commercial integrated photonic devices and hold promising application prospects in quantum information processing and quantum light sources.